Modeling a slab above another slab

[Anonymous]

Hi,

This is my first post in here.

I work at a telecom carrier and we install GSM towers on top of buildings. We usually install the towers on beams system fixed to building columns without any issues.

I am facing now a new situation where we need to install a tower on a building that doesn't have concrete columns. That means that there isn't any members that can take tension/uplift. The building is supported on old stone walls; which means only compression/downward forces can be taken.

As you know, the tower supporting lots of antennas will generate compression/tension forces at its legs when submitted to wind.

I am thinking of installing the 12m tower on a concrete foundation on an intersection of stone walls. Likewise most of the weight can be taken on these walls and part of it goes to roof slab. 

Most of the tension forces will be negated by the weight of the concrete foundation. The remaining tension forces shall be taken by small dowels (steel rebars that i'm willing to distribute between the roof slab and the concrete foundation.

The situation is tricky.

Think of it as a tower+foundation on ground! We usually set the soil parameters under the foundation while of course setting uplift upwards (because of wind and the huge overturning moment generated).

The model hence run on a non-linear anaylisis.

In our case, instead of the soil, we have the roof slab. And instead of the soil not taking any kind of tension/uplift, we are allowing some uplift/tension that should be taken by the dowels.

The problem is the following:

I want to model, if possible, the whole system: Roof slab + tower foundation/slab above it + tower above the foundation!

As far as I know, there is no way to model a slab above another slab in finite element analysis! 

I tried the following 3 models:

1- One model of: roof slab + tower directly fixed to it + (tower foundation as self-weight load).

Problem with this model is that it generates huge concentrated stresses on the roof slab under tower legs.

This is not real behavior, since tower efforts will be distributed on a thick concrete foundation before reaching the roof slab.

So with this model, I don't have the exact behavior of the roof slab. Besides I cannot dimension the dowels that will connect the roof slab to the tower foundation.

2- One model of: roof slab + tower directly fixed to it + part of roof slab at much higher thickness (roof slab th + foundation th).

Problem with this model is that it change the overall behavior. Besides I cannot check the effect of the tower on the roof slab; especially steel reinforcement.

So with this model, I don't have the exact behavior of the roof slab. Besides I cannot dimension the dowels that will connect the roof slab to the tower foundation.

3- One model of: tower + its foundation on a mesh of pinned supports (distributed each 50cm both directions for example). 

With this model I can fine tune the tower foundation size in a way to generate acceptable tension forces to be taken by dowels.

So with this model I can get the foundation size and the dowels, but not its effect on the roof slab.

Taking each support/dowel reaction from model 3 and applying it on model 1 of the roof slab will be a big hassle since there are 12 wind directions to consider (triangular tower) and lots of supports. 

I hope you could still follow me and you understood the situation.

Thanks for reading all this. Do you think you can help?!

[Artur.Kosakowski · Accepted solution]

Have you considered modelling two panels one above the other connected with number of vertical bars defined as compression only elements or with unidirectional release at one end? 

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.

[Artur.Kosakowski]

You may find this topic helpful:

http://forums.autodesk.com/t5/robot-structural-analysis/support-conditions-for-sandwich-type-base-sl...

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.